Fixed wireless access systems are currently employed for local telecommunication networks, such as the IONICA system. Known systems comprise an antenna and decoding means which are located within a subscriber's premises, for instance adjacent a telephone. The antenna receives the signal and provides a further signal by wire to a decoding means. Thus subscribers are connected to a telecommunications network by a radio link in place of the more traditional method of copper cable. Such fixed wireless access systems will be capable of delivering a wide range of access services from POTS (public operator telephone service), ISDN (integrated services digital network) to broadband data. The radio transceivers at the subscribers premises communicate with a base station, which provides cellular coverage over, for example, a 5 km radius in urban environments. A typical base station will support 500-2000 subscribers. Each base station is connected to a standard PSTN switch via a conventional transmission link/network.
When a fixed wireless access telecommunications system is originally deployed, then a base station of a particular capacity will be installed to cover a particular area. The capabilities of the base station will be commensurate with the anticipated coverage and capacity requirement. Subscribers antennas will be mounted outside, for instance on a chimney and upon installation will be directed towards the nearest base station or repeater antenna (any future reference to a base station shall be taken to include a repeater). In order to meet the capacity demand, within an available frequency band allocation, fixed wireless access systems divide a geographic area to be covered into cells. At the centre of each cell is a base station through which the subscribers stations communicate; the distance between the cells being determined such that co-channel interference is maintained at a tolerable level. When the antenna on the subscriber premises is installed, an optimal direction for the antenna is identified using monitoring equipment. The antenna is then mounted so that it is positioned towards the optimal direction. The antenna is then fixed in this position and can only be moved by reinstalling it.
A potential problem with this type of directed, fixed antenna is that the "optimal direction" for the antenna may change over time. For example, if the antenna is installed during the winter, summer foliage may later obscure the antenna. Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems. These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions. Since the various components arrive from different directions, there is also a Doppler spread in the received spectrum. Correct alignment and installation of a fixed wireless access subscriber terminal towards a geographically proximate base station is essential for the correct performance of the network.
Another possibility is that the position of the nearest base station may change, for example, as more base stations are installed when the number of subscribers increases. In the situation that the subscriber would like to receive signals from different base stations at different times (for example, from two different geo stationary satellites), then the subscriber would need to change the direction of the antenna. This can be done by manually moving the antenna, or installing two antennas, although both these options are inconvenient and time consuming.
A further problem concerns situations when the subscriber would like to communicate, with non geo-stationary satellites. For example, with low earth orbiting LEO or medium earth orbiting MEO satellite systems, where the satellite direction changes with time and fast "hand over" between satellites is required in order to provide an uninterrupted service. This could be achieved using two antennas, each mechanically steered. However, this is expensive and it is often more unsightly to site two antennas on a building rather than just one. For the purposes of this application, the term "fixed" should not be understood as being a limitation upon the direction of the antenna. A similar situation could exist in a satellite communications environment when, for example, one might wish to watch a television programme provided by one satellite whilst videoing a programme from another satellite. This is not possible using a single directed fixed antenna (with a single aperture) as is typically used in such systems. In order to receive signals from two different directions simultaneously, the subscriber would need to install two antennas which increases the costs involved.
U.S. patent application Ser. No. 08/758,989, in the name of Northern Telecom Ltd, describes a system which allows a single directional antenna beam to be aligned for one direction. The specification describes an antenna assembly for a fixed wireless access system, where the antenna direction is adjustable using a motorised, mechanical positioning system. However, this type of system can be expensive to manufacture and install. Further, in a LEO or MEO environment, handover would not be seamless.
Maddocks and Smith describe a system for allowing a single antenna beam to be aligned for one direction in their paper, "Flat-plate steerable antennas for satellite communications and broadcast reception" IEE Proceedings H, Vol. 138, No. 2, April 1991. This describes a flat plate antenna assembly that is designed to be mounted flush on a wall. The antenna assembly can be mechanically rotated in the plane of the assembly as one degree of freedom. A Rotman lens beamformer is provided to provide the antenna beam with a second degree of freedom. However, this system only allows a single beam to be aligned for one direction. Also, a polarisation converter must be used which adds to the cost and bulk of the antenna assembly.